The last phase, when the core fuses silicon, lasts less than three weeks. As silicon fusion ends, an Earth-sized iron-nickel core about 1. Neutrinos stream from the core. The electrons transform some protons into neutrons.
Both processes — streaming neutrinos and squeezing protons and electrons together — remove pressure that supports the star. In half a second or less, it transforms from an Earth-sized stellar core to a hot, dense proto-neutron star just 19 miles 30 kilometers across. When the central density reaches about twice that of an atomic nucleus, the core stiffens and rebounds thanks to a repulsive component in the strong nuclear force.
The central mystery of core-collapse supernovae is how this situation ever can turn itself around. They revealed fluid instabilities and turbulence that promised to aid the stalled shock. In , Burrows and his colleagues discovered a potentially important alternative energy source in collapsing stars: sound waves.
As matter streams onto the proto-neutron star, turbulence around the core sets it oscillating at around hertz — musically, about F above middle C. Acoustic waves radiate back into the collapsing envelope. While the energy from neutrinos is far greater, only a fraction of it becomes deposited in the stalled shock, whereas matter absorbs sound almost completely. How important this process is remains an open question.
Large-scale computer simulations are also providing new insights into how white dwarfs, the end state of low-mass stars, destroy themselves as type Ia supernovae. Brighter and more uniform than core-collapse explosions, type Ia events are important probes of the distant universe. The discoveries of dark energy and cosmic acceleration add urgency to deciphering how they work.
Place a white dwarf in close proximity to a normal star, and the dwarf can gain mass until it nears the 1. As a white dwarf tips the scale toward 1. Before , no one could figure out how to make a carbon-oxygen star detonate, so theorists first invoked turbulent thermonuclear fusion. These simulations failed to match the energy and element mix of type Ia blasts. Models that followed a period of turbulent burning with a detonation better matched reality, but theorists simply decided where and when the explosion would occur and inserted it into the simulation.
In , a team led by Alan Calder, then at the University of Chicago, including Lamb, stumbled onto a way to blow up a white dwarf. Thanks to the U. After ignition, a narrow front of nuclear flame expanded through the star, leaving behind a billion-degree ash bubble.
The nuclear ash erupts, moving at around 6. As it does so, it plows up cooler, unfused surface material. The superheated ash-cloud wraps around the white dwarf and meets itself at the point opposite its breakout. And what's really cool about that is that there's so many of those elements in our bodies that literally came from exploding stars.
ERIC: That's so cool that we are made of star explosion debris. We also get a lot of questions about what is left over after a star explodes. So what happens to the parts that aren't blown away?
So that core will continue to collapse under its own gravity and it can form one of two objects. It can become something called a neutron star or it can form into a black hole. Neutron star sounds like kind of ridiculously awesome, like it's from a bad science fiction movie.
Where does it get its name from? So you have all of this material that is squished into the size of really a small city.
You have so much mass squished in there that protons and electrons that make up atoms actually end up squishing together because of how high the density is. And so you no longer have positively or negatively charged particles.
You now just have neutrons. So it's just a giant ball of neutrons, essentially. It would be like trying to squish two of the sun into the size of Boston. ERIC: That's intense. The other outcome you mentioned is a black hole and we get so many questions about those that we'll have to spend an entire episode at least talking about them.
But can you tell us what is a black hole and why do we call it that? So black holes are even denser than neutron stars. A black hole happens when all of that material that makes up the core of an exploded star collapses into an infinitely small point.
So you can think of taking 10 times the mass of our sun and squishing it into a period at the end of a sentence, but even smaller than that. And so these objects, these black holes, have so much mass in such a small amount of space that their gravity is immense. If even light gets too close to a black hole, it can't escape. In order to escape a black hole you'd need to be moving faster than the speed of light. And we don't know of anything that can do that. So there's no light that's actually being reflected out of a black hole.
That's why we call them black. ERIC: So even though their gravity is incredibly strong, they're not actually sucking things up. It's a really beautiful deep sky object and Mahiro wanted to know why does it look like an explosion?
During that time it was recorded that it looked like a new star in the sky, but when we look now with telescopes we see all of this material that is left over from an exploded star. We often get asked if any of those could explode any time soon.
And if they did, what would it look like from Earth and would it destroy us? And I get this one a lot, especially in the planetarium. But yeah, I mean there are a few stars that we can see with our eyes that are nearing the end of their lives and that are massive enough to go supernova. A couple that come to mind our Betelgeuse in the constellation of Orion, or Antares in the constellation of Scorpius.
And both of these stars are red super giants that are nearing the end of their lives, but it's hard to tell exactly when they are going to go supernova. When astronomers say soon they generally mean within , years or so.
This causes the pressure to drop. Gravity wins out, and the star suddenly collapses. Imagine something one million times the mass of Earth collapsing in 15 seconds! The collapse happens so quickly that it creates enormous shock waves that cause the outer part of the star to explode!
Usually a very dense core is left behind, along with an expanding cloud of hot gas called a nebula. A supernova of a star more than about 10 times the size of our sun may leave behind the densest objects in the universe— black holes. The Crab Nebula is the leftover, or remnant, of a massive star in our Milky Way that died 6, light-years away. Astronomers and careful observers saw the supernova in the year Hester and A.
Loll Arizona State University. A second type of supernova can happen in systems where two stars orbit one another and at least one of those stars is an Earth-sized white dwarf. A white dwarf is what's left after a star the size of our sun has run out of fuel.
If one white dwarf collides with another or pulls too much matter from its nearby star, the white dwarf can explode. In this illustration, a white dwarf pulls matter from a companion star. Eventually, this will cause the white dwarf to explode. Image credit: STScI.
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